Materials testing machines for simulating actual forces upon materials, such as the forces of vehicular traffic upon an asphalt surface of a road bed, have been devised to produce a sample of material which evidences the physical effects of repeated loading by for example compression, compaction, shear strain, plane-strain, and thermal reactivity. Such machines typically include a material holding mold which is inserted into a chamber or carriage which positions the mold for insertion of a ram into the mold cavity to compress the material in the mold. The mold may be gyrated about a small angle relative to the vertical axis of the ram, by motion of the mold carriage, as the material is compressed by the ram to simulate actual forces on the material in the application environment.
U.S. Pat. No. 2,972,249 discloses a kneader compactor which uses opposed plungers to compress materials within a mold chuck mounted for gyratory oscillation to produce kneading stresses in the material in the mold. Gyration of the mold as the material is compressed produces relative motion of particles of the material which simulates the physical response of asphalt material to vehicle load forces.
U.S. Pat. Nos. 4,942,768 and 5,036,709 disclose a paving material testing machine in which paving material to be tested is placed in a mold held by a chuck which is gyrated about a vertical axis while the material is compressed in the mold from the bottom by a hydraulically driven ram. As the mold chuck is rotated, a portion of the mold chuck in contact with the mold dynamically influences the axially adjustable chuck so that deformation of the material within the mold induced by the gyration changes the angle of gyration. This subjects the material in the mold to a gyratory kneading action analogous to the forces exerted by vehicles moving over asphalt surfaces. The machine also performs cyclic vertical loading by timed control of predetermined applied forces of the hydraulic ram upon the sample to simulate the flexing forces of vehicle tires rolling upon the asphalt surface.
Accurate calibration, control and monitoring of the compressive load of the ram upon a sample within the mold as the mold is gyrated is critical to obtaining accurate test results, i.e., compacted specimens which have specified densities, substantially uniform alignment of aggregate materials mixed with the specimen, and elastic properties which closely approximate real world applications. Precise control of the linear travel and compressive force of hydraulically driven rams in compaction devices requires the use of comparatively expensive control components. Also, hydraulic systems are heat sensitive and require frequent maintenance of seals and fluid.
The angle of gyration of the mold during compaction is also a critical factor which determines the amount of kneading action with resultant shear stress and strain of the material within the mold. Although the prior art devices induce kneading action of the material within the mold, the angle of gyration as determined by the density and flow of the material adds a variable which complicates accurate interpretation of the test results. Prior gyratory compaction testing machines do not provide for a precisely fixable and adjustable angle of gyration.
Automated control and safety of operation of gyratory compactors, each vital to obtaining accurate tests results without extensive operator training, are features not adequately addressed in the design of prior machines.